The present invention relates to a method for fabricating a Group III nitride semiconductor substrate for use in a semiconductor laser which emits light at a shorter wavelength such as blue or purple light and in a transistor operating at a high temperature.
A Group III nitride semiconductor represented by AlxGayIn1-x-yN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61) (hereinafter referred to as a Group III nitride semiconductor) is a material used in an optical device which emits light at a wavelength ranging in color from red to ultraviolet so that the applications thereof to a light emitting device and a light receiving device are expected. Thus far, a Group III nitride semiconductor film of relatively high quality has been formed conventionally by crystal growth on a sapphire substrate.
However, since the Group III nitride semiconductor film and the sapphire substrate do not lattice-match each other, the Group III nitride semiconductor film contains numerous crystal defects so that a device using a Group III nitride semiconductor has degraded properties.
If the Group III nitride semiconductor film formed on the sapphire substrate is used in a semiconductor laser or transistor, all electrodes should be formed on the Group III nitride semiconductor film since the sapphire substrate is a so-called insulating substrate which does not allow the passage of electricity. This has complicated a fabrication process for a device composed of a Group III nitride semiconductor and reduced the production yield thereof.
To increase the fabrication yield and performance of the device using the Group III nitride semiconductor, a group III nitride semiconductor substrate (especially a GaN substrate) having a high quality and a large area has been in growing demand. Under such circumstances, there have been proposed various methods in each of which a Group III nitride semiconductor film is grown on a substrate made of a different type of material (sapphire substrate or the like) and then the substrate made of the different type of material is removed.
For example, there has been known a conventional method in which a sapphire substrate and a GaN film are separated from each other by irradiation with an intense laser beam (Michael K. Kelly et al., Japanese Journal of Applied Physics Vol.38 p.L217-L219, 1999). A description will be given herein below to the conventional method with reference to FIGS. 11A to 11C, which are cross-sectional views illustrating the process steps of the conventional method.
In the step shown in FIG. 11A, a GaN layer 102 with a thickness of 200 to 300 xcexcm is formed on a sapphire substrate 101 having a diameter of 2 inches and a C surface as a principal surface by using hydride vapor phase epitaxy (hereinafter referred to as HVPE).
Next, in the step shown in FIG. 11B, the sapphire substrate 101 formed with the GaN layer 102 is retrieved from a HVPE reactor. Then, the lower surface of the GaN layer 102a is entirely scanned with a laser beam at a wavelength of 355 nm applied thereto through the sapphire substrate 101. The arrow in the drawing represents the laser beam. As a result, heat is generated at the portion irradiated with the laser beam to decompose a lower portion of the GaN layer 102.
Next, in the step shown in FIG. 11C, the sapphire substrate 101 and the GaN layer 102 are separated from each other so that an independent GaN substrate 102a is obtained.
However, the foregoing method has the following problems.
The respective thermal conductivities of GaAs and InP which are representatives of Group III-V compound semiconductors are 0.54 W/cmK and 0.68 W/cmK, while the thermal conductivity of Si used for a submount for heat dissipation is 1.5 W/cmK.
On the other hand, the thermal conductivity of GaN is 1.3 W/cmK. From a comparison between the thermal conductivity of GaN and the thermal conductivities of the foregoing materials, it will be understood that GaN is a material which readily conducts heat. In accordance with the conventional method which irradiates GaN with the laser beam, the heat generated in the lower portion of the GaN layer 102 through the absorption of the laser beam is likely to be diffused. This causes the problem that an amount of heat required to completely decompose the portion of the GaN layer 102 irradiated with the laser beam in the step shown in FIG. 11B is insufficient and the efficiency with which GaN is decomposed is reduced. If the efficiency with which GaN is decomposed is reduced, the sapphire substrate 101 and the GaN layer 102 should be separated from each other by increasing the number of times that the GaN layer 102 is scanned with the laser beam and thereby supplying a sufficient amount of heat to completely decompose the portion of the GaN layer 102 irradiated with the laser beam. Accordingly, the time required to perform the step shown in FIG. 11B is increased so that productivity is lowered.
Since GaN and sapphire do not lattice-match each other, the GaN layer 102 contains numerous crystal defects and distortions. As a result, an impact resulting from the release of a stress when GaN is decomposed may cause a fracture in the GaN substrate 102a obtained. If the number of scannings is increased, the probability of a fracture occurring in the GaN substrate 102a is increased.
Even if the GaN substrate 102a undergoes, a crack may remain within the GaN substrate 102a. If a device such as a light-emitting diode or a laser diode is fabricated by using a GaN substrate 102a having a crack remaining therein, the crack causes a leakage current and reduces the reliability of the device.
The present invention has been achieved to solve the foregoing problems and it is therefore an object of the present invention to provide a high-quality Group III nitride semiconductor substrate.
A method for fabricating a Group III nitride semiconductor substrate according to the present invention comprises the steps of: (a) preparing a substrate; (b) forming, on the substrate, a first semiconductor layer composed of a Group III nitride semiconductor; (c) forming, on the first semiconductor layer, a heat diffusion suppressing layer lower in thermal conductivity than the first semiconductor layer; (d) forming, on the heat diffusion suppressing layer, a second semiconductor layer composed of a Group III nitride semiconductor; and (e) irradiating the first semiconductor layer through the substrate with a light beam transmitted by the substrate and absorbed by the first semiconductor layer to decompose the first semiconductor layer.
In accordance with the present invention, the heat diffusion suppressing layer lower in thermal conductivity than the first semiconductor layer is formed between the first and second semiconductor layers to suppress the diffusion of heat generated through the absorption of the light beam by the first semiconductor layer. Accordingly, the majority of the generated heat contributes to the decomposition of the first semiconductor layer so that the first semiconductor layer is decomposed efficiently. Even if the number of scannings with the light beam is smaller than in the conventional embodiment, heat required to decompose the first semiconductor layer completely can be supplied in a sufficient amount so that productivity is increased. Since the number of scannings with the light beam is smaller than in the conventional embodiment, the probability of a fracture occurring in the Group III nitride semiconductor substrate separated from the second semiconductor layer can be reduced.
The Group III nitride semiconductor composing the heat diffusion suppressing layer may be lower in thermal conductivity than the Group III nitride semiconductor composing the first semiconductor layer.
The heat diffusion suppressing layer may be composed of a semiconductor represented by InxGa1-xN (0 less than xxe2x89xa61).
Preferably, the step (c) includes forming the heat diffusion suppressing layer and then forming an opening extending through the heat diffusion suppressing layer and reaching the first semiconductor layer.
In the arrangement, the Group III nitride semiconductor crystal composing the second semiconductor layer grows along the upper surface of the heat diffusion suppressing layer during the formation of the second semiconductor layer. As a result, dislocation occurring in the first semiconductor layer is hardly propagated to the second semiconductor layer so that the number of spots undergoing dislocation in the second semiconductor layer is significantly reduced. This allows the fabrication of a Group III nitride semiconductor substrate with an excellent crystalline property.
The heat diffusion suppressing layer may be composed of a metal.
The heat diffusion suppressing layer may be composed of at least one metal selected from the group consisting of Ni, Pt, and Ti.
The heat diffusion suppressing layer may be composed of a dielectric material.
The heat diffusion suppressing layer may be composed of at least one dielectric material selected from the group consisting of a silicon dioxide film and a silicon nitride film.
Preferably, the method further comprises, after the step (e), the step of: (f) removing the heat diffusion suppressing layer.
The step (f) may include removing the heat diffusion suppressing layer by etching.
The step (f) may include removing the heat diffusion suppressing layer by polishing.
Preferably, the substrate is lower in thermal conductivity than the group III nitride semiconductor composing the first semiconductor layer.
The arrangement suppresses conduction of heat to the substrate and allows a large amount of heat to contribute to the decomposition of the first semiconductor layer.
Another method for fabricating a Group III nitride semiconductor substrate according to the present invention comprises the steps of: (a) preparing a substrate; (b) forming, on the substrate, a first semiconductor layer composed of a Group III nitride semiconductor; (c) forming, on the first semiconductor layer, a light reflecting layer; (d) forming, on the light reflecting layer, a second semiconductor layer composed of a Group III nitride semiconductor; and (e) irradiating the first semiconductor layer through the substrate with a light beam transmitted by the substrate and absorbed by the first semiconductor layer to decompose the first semiconductor layer, the light reflecting layer reflecting the light beam applied in the step (e).
According to the present invention, the laser beam which has not been absorbed by the lower portion of the first semiconductor layer during light beam application is reflected back by the light reflecting layer to contribute the thermal decomposition of the first semiconductor layer. This reduces the threshold of the irradiation energy of the light beam required to thermally decompose the first semiconductor layer to a value lower than in the conventional embodiment. As means for lowering the threshold value of the irradiation energy of the light beam, the diameter of the light beam can be increased. Even if the number of scannings with the light beam is smaller than in the conventional embodiment, heat required to decompose the first semiconductor layer completely can be supplied in a sufficient amount so that productivity is increased. Since the number of scannings with the light beam is smaller than in the conventional embodiment, the probability of a fracture occurring in the Group III nitride semiconductor substrate separated from the second semiconductor layer can be reduced.
Preferably, the first semiconductor layer has a first layer composed of a Group III nitride semiconductor having a band gap smaller than energy of the light beam and a second layer composed of a Group III nitride semiconductor having a band gap larger than the energy of the light beam, the second layer being formed on the first layer.
In the arrangement, the light beam is not absorbed by the second layer. As a result, attenuation is suppressed when the light beam which has not been absorbed by the lower portion of the first semiconductor layer is reflected by the light reflecting layer. This provides a higher utilization of the irradiation energy of the light beam contributing to the thermal decomposition of the first semiconductor layer than in the conventional embodiment.
Preferably, the step (c) includes forming the light reflecting layer and then forming an opening extending through the light reflecting layer and reaching the first semiconductor layer.
In the arrangement, the Group III nitride semiconductor crystal composing the second semiconductor layer grows along the upper surface of the heat diffusion suppressing layer during the formation of the second semiconductor layer. As a result, dislocation occurring in the first semiconductor layer is hardly propagated to the second semiconductor layer so that the number of spots undergoing dislocation in the second semiconductor layer is significantly reduced. This allows the fabrication of a Group III nitride semiconductor substrate with an excellent crystalline property.
The light reflecting layer may be composed of a dielectric material.
The light reflecting layer may be a multilayer film composed of silicon dioxide films and titanium oxide films which are alternately stacked in layers.
Still another method for fabricating a Group III nitride semiconductor substrate according to the present invention comprises the steps of: (a) preparing a substrate; (b) forming a light scattering portion within the substrate; (c) forming, on the substrate, a semiconductor layer composed of a Group III nitride semiconductor; and (d) irradiating the semiconductor layer through the substrate with a light beam transmitted by the substrate and absorbed by the semiconductor layer to decompose a lower portion of the semiconductor layer.
In accordance with the present invention, the light beam is scattered by the light scattering portion to have a larger diameter upon reaching the semiconductor layer. Even if the number of scannings with the light beam is smaller than in the conventional embodiment, heat required to decompose the first semiconductor layer completely can be supplied in a sufficient amount so that productivity is increased. Since the number of scannings with the light beam is smaller than in the conventional embodiment, the probability of a fracture occurring in the Group III nitride semiconductor substrate separated from the second semiconductor layer can be reduced.
The step (b) may include implanting ions into the substrate to form the light scattering portion within the substrate.
The step (c) may be performed after the step (a) and the step (b) may include implanting the ions into the substrate through the semiconductor layer to form the light scattering portion within the substrate, the method further comprising, between the steps (b) and (d), the step of: forming, on the semiconductor layer, another semiconductor layer composed of a Group III nitride semiconductor.
The step (b) may include forming, as the light scattering portion, a plurality of depressed portions in a lower portion of the substrate.
The plurality of depressed portions may be formed with application of a plasma.